Electrically driven hammers are known in which a driving member in the form of a flying mass is reciprocally driven in a piston, and impact of the flying mass against the end of the piston imparts a hammer action to a bit of the hammer. Such an arrangement is disclosed in European patent application EP1252976 and is shown in FIG. 1.
Referring in detail to FIG. 1, the prior art demolition hammer comprises an electric motor 2, a gear arrangement and a piston drive arrangement which are housed within a metal gear housing 5 surrounded by a plastic housing 4. A rear handle housing incorporating a rear handle 6 and a trigger switch arrangement 8 is fitted to the rear of the housings 4, 5. A cable (not shown) extends through a cable guide 10 and connects the motor to an external electricity supply. When the cable is connected to the electricity supply when the trigger switch arrangement 8 is depressed, the motor 2 is actuated to rotationally drive the armature of the motor. A radial fan 14 is fitted at one end of the armature and a pinion is formed at the opposite end of the armature so that when the motor is actuated the armature rotatingly drives the fan 14 and the pinion. The metal gear housing 5 is made from magnesium with steel inserts and rigidly supports the components housed within it.
The motor pinion rotatingly drives a first gear wheel of an intermediate gear arrangement which is rotatably mounted on a spindle, which spindle is mounted in an insert to the gear housing 5. The intermediate gear has a second gear wheel which rotatingly drives a drive gear. The drive gear is non-rotatably mounted on a drive spindle mounted within the gear housing 5. A crank plate 30 is non-rotatably mounted at the end of the drive spindle remote from the drive gear, the crank plate being formed with an eccentric bore for housing an eccentric crank pin 32. The crank pin 32 extends from the crank plate into a bore at the rearward end of a crank arm 34 so that the crank arm can pivot about the crank pin 32. The opposite forward end of the crank arm 34 is formed with a bore through which extends a trunnion pin 36 so that the crank arm 34 can pivot about the trunnion pin 36. The trunnion pin 36 is fitted to the rear of a piston 38 by fitting the ends of the trunnion pin 36 into receiving bores formed in a pair of opposing arms which extend to the rear of the piston 38. The piston is reciprocally mounted in cylindrical hollow spindle 40 so that it can reciprocate within the hollow spindle. An O-ring seal 42 is fitted in an annular recess formed in the periphery of the piston 38 so as to form an airtight seal between the piston 38 and the internal surface of the hollow spindle 40.
When the motor 2 is actuated, the armature pinion rotatingly drives the intermediate gear arrangement via the first gear wheel and the second gear wheel of the intermediate gear arrangement rotatingly drives the drive spindle via the drive gear. The drive spindle rotatingly drives the crank plate 30 and the crank arm arrangement comprising the crank pin 32, the crank arm 34 and the trunnion pin 36 converts the rotational drive from the crank plate 30 to a reciprocating drive to the piston 38. In this way the piston 38 is reciprocatingly driven back and forth along the hollow spindle 40 when the motor is actuated by a user depressing the trigger switch 8.
The spindle 40 is mounted in magnesium housing 42 from the forward end until an annular rearward facing shoulder (not shown) on the exterior of the spindle butts up against a forward facing annular shoulder (not shown) formed from a set of ribs in the interior of the magnesium casing 42. The ribs enable air in the chamber surrounding the spindle 40 to circulate freely in the region between ram 58 and beat piece 64. An increased diameter portion on the exterior of the spindle fits closely within a reduced diameter portion on the interior of the magnesium casing 42. Rearwardly of the increased diameter portion and the reduced diameter portion an annular chamber is formed between the external surface of the spindle 40 and the internal surface of the magnesium casing 42. This chamber is open at its forward and rearward ends. At its forward end the chamber communicates via the spaces between the ribs in the magnesium casing with a volume of air between the ram 58 and the beat piece 64. At its rearward end the chamber communicates via the spaces between the ribs 7 and the recess of the gear casing 5 with a volume of air in the gear casing 5.
The volume of air in the gear casing 5 communicates with the air outside of the hammer via a narrow channel 9 and a filter 11. The air pressure within the hammer, which changes due to changes in the temperature of the hammer, is thus equalized with the air pressure outside of the hammer. The filter 11 also keeps the air within the hammer gear casing 5 relatively clean and dust free.
A ram 58 is located within the hollow spindle 40 forwardly of the piston 38 so that it can also reciprocate within the hollow spindle 40. An O-ring seal 60 is located in a recess formed around the periphery of the ram 58 so as to form an airtight seal between the ram 58 and the spindle 40. In the operating position of the ram 58 (shown in the upper half of FIG. 1), with the ram located behind bores 62 in the spindle, a closed air cushion is formed between the forward face of the piston 38 and the rearward face of the ram 58. Reciprocation of the piston 38 thus reciprocatingly drives the ram 58 via the closed air cushion. When the hammer enters idle mode (i.e. when the hammer bit is removed from a work piece), the ram 58 moves forwardly, past the bores 62 to the position shown in the bottom half of FIG. 1. This vents the air cushion and so the ram 58 is no longer reciprocatingly driven by the piston 38 in idle mode, as is known to persons skilled in the art.
However, known hammer drills of this type suffer from the drawback that the hammer action generates significant vibrations, which can be harmful to users of the apparatus, and can cause damage to the apparatus itself.
It is known to reduce the effect of vibrations on users of power tools by providing absorbent material around handles of the tool, the absorbent material acting as passive vibration damping material. However, the effectiveness of such materials in reducing the transmission of vibrations to the user of the apparatus is limited.
Active vibration reduction apparatus are known in which rotatable masses are driven about respective axes of rotation, the centres of mass of the rotatable masses being spaced from the axes of rotation such that rotation of the masses about the axes of rotation generates vibrations. By controlling the frequency of rotation of the masses, and the relative phases between the centres of mass of the masses, vibrations can be generated which can be used to counteract unwanted vibrations, for example in diesel motors. Such arrangements are disclosed in FR 2606110, WO 88/06687, FR 2550471, EP 0840191, EP 0505976 and EP 0337040. However, it has not to date been considered feasible to apply such techniques to the reduction of unwanted vibrations generated in power tools.
DE 3427342 discloses a hammer drill in which a wobble plate drive mounted to a shaft causes reciprocating movement of a flying mass to impart impacts to a bit of the drill. The wobble plate drive has a finger engaging a piston forming part of the hammer mechanism such that rotation of a shaft to which the wobble plate is mounted causes reciprocating movement of the piston, and a countermass arranged on an opposite side of the shaft to the finger to partially counteract vibrations produced by the apparatus. However, this drill suffers from the drawback that its vibration reducing abilities and compactness are limited.
GB 2256905 discloses a reciprocating saw in which first and second wobble plates reciprocatingly drive a saw blade and counterweight respectively to minimise vibrations produced by the saw. However, this saw suffers from the drawback that the provision of a pair of wobble plates makes it difficult to construct the saw compactly.
Preferred embodiments of the present invention seek to overcome the above disadvantages of prior art power tools.